1. Field of the Invention
This invention relates to an exposure device in an electronic still camera provided with a solid state image pickup device having an electronic shutter function such as CCD.
2. Related Background Art
In an electronic still camera using a solid stage image pickup device having an electronic shutter function, for example, CCD (such as an interline transfer CCD or a frame transfer CCD), the following systems are conceivable as a system for finding a proper exposure value (an aperture value and an exposure time) from the photometering output of an object using the CCD.
A first system is such that before a photographing starting switch (a release switch) is depressed, that is, by the operation of preparing for photographing, such as the half-depressing operation of a release button or the operator touching the release button by his finger, a power source is supplied to the CCD and the circuits around it to video-operate the CCD, and the average value of output levels of one field or any period within one field is found as the photometering output of an object and a proper exposure value (an aperture value and an exposure time) is found from this average value.
A second system is such that after the release switch is depressed (after the half-depressing operation of the release button), a power source is supplied to the CCD and the circuits around it to video-operate the CCD, and the average value of the output levels of one field or any period within one field is found as the photometering output of an object and a proper exposure value (an aperture value and an exposure time) is found from this average value.
However, an exposure control device in which a proper exposure value (an aperture value and an exposure time) is found with the output signal obtained by such video operation of the CCD as the photometering output of the object suffers from the following problems.
According to the above-described first system, the power source must be supplied to the CCD and the circuits around it from the stage of the operation of preparing for photographing before the release switch is depressed, and the CCD and the circuits around it are one of the greatest power consuming sources and therefore, this system is not preferable from the viewpoint of the consumption efficiency of the power source.
Also, the reliable range of the exposure value calculated from the picture signal output of the CCD is of the order of .+-.2 EV for the proper exposure value, from the limitation in the charge transfer capability or the like of the CCD. When the quantity of light introduced at the aperture value and exposure time set at first deviates from the proper exposure value by .+-.2 EV or more, the aperture or the exposure time must be controlled after photometering and a photometering operation must be again effected and this operation must be repeatedly effected until the deviation is within .+-.2 EV from the proper exposure value, and at a point of time whereat the deviation has been within .+-.2 EV, this output signal is used to obtain proper exposure. Accordingly, there is the undesirable possibility that much time is taken for the photometering for obtaining proper exposure.
Further, where the structure of an electronic still camera is made into the type of a single-lens reflex camera, when an attempt is made to effect photo-metering by the output of a CCD while an object is observed through a photo-taking lens and a finder, it is necessary to cause the object light for photometering to enter the CCD provided behind a quick return mirror and therefore, the structure of the quick return mirror must be constructed of a special member such as a half-mirror, and this is disadvantageous.
On the other hand, in the aforedescribed second system, as in the first system, if the quantity of light introduced at an aperture value and an exposure time set at first after the release switch is depressed deviates from a proper exposure value by .+-.2 EV or more, the aperture value or the exposure time is controlled and the photometering operation is repeated again and thus, there is the undesirable possibility that very much time is required from after the release switch is depressed until exposure is actually started, and this is a problem as viewed from the viewpoint of operability.
Also, there is no means for knowing the exposure value (the aperture value and the exposure time) before the release switch is depressed and therefore, it is necessary to provide a new photometering sensor such as SPD discretely for the display of the exposure value, and this leads to an increased number of parts and a complicated mechanism.